UNIVERSITY  OF  CALIFORNIA  AGRICULTURAL  EXPERIMENT  STATION 

COLLEGE  OF  AGRICULTURE  Benj.  Ide  Wheeler,  President 

BERKELEY,  CALIFORNIA  Thomas  Forsyth  Hunt,  Dean  and  Director 


CIRCULAR  No.  98. 


PLOWING  AND  CULTIVATING  SOILS  IN  CALIFORNIA. 

By   ClTAS.   B.    LlPMAN. 

OBJECTS  OF  PLOWING  AND  CULTIVATING. 

Careful  observation  and  experiment  have  amply' shown  that  plant 
roots  must  have  air  to  develop  successfully.  They  have  also  shown  air 
to  be  essential  for  the  life  of  all  of  the  beneficial  soil  bacteria  upon  which 
we  depend  for  the  solution  of  plant  food  in  the  soil,  and  for  the  addition 
of  nitrogen  from  the  air.  In  other  words,  the  most  vital  factors  in 
successful  plant  growth  are  dependent  on  a  good  supply  of  air  in  the 
soil.  Such  a  supply  of  air  can  best  be  furnished  and  maintained 
through  plowing  and  cultivating.  Besides  accomplishing  the  objects 
above  named  tillage  operations  are  important,  because  they  make  pos- 
sible the  turning  under  for  humus  formation  of  organic  manures,  and 
for  turning  under  other  forms  of  organic  matter  and  other  fertilizers. 
They  keep  down  weeds,  they  make  a  larger  reservoir  for  water,  and  they 
prevent  evaporation  of  water  from  soils. 

REASONS  FOR  THE  NEED  OF  DEEP  PLOWING  IN  CALIFORNIA. 

California's  celebrated  deep  soils,  and  the  need  for  deeply  plowing 
them,  have  become  understood  through  a  careful  study  of  the  climatic 
conditions  of  this  and  other  arid  regions,  or  those  in  which  every  year 
has  a  long  period  of  several  months  of  drouth.  The  most  striking  fact 
learned  in  these  studies  is  that  clay  forms  very  slowly  under  arid  con- 
ditions. It  is  produced  from  feldspar  minerals  when  certain  elements 
(like  sodium  and  potassium)  are  leached  out  of  them.  It  is  easy  to 
reason,  therefore,  that  if  clay  can  only  be  formed  from  the  leaching 
action  of  water  on  feldspar  minerals  through  which  certain  elements  are 
washed  out,  that  where  there  is  not  enough  water  to  leach  the  minerals, 
the  elements  spoken  of  will  not  be  washed  out,  and  therefore  clay  will 
not  be  formed  at  all  or  only  very  slowly.  But  if  clay  forms  very  slowly 
there  will  not  be  anything  in  the  soil  to  exercise  a  cementing  action  on 
the  sand  and  silt  particles.  Further,  if  the  soil  particles  are  not 
cemented  they  must  remain  loosely  scattered,  and,  therefore,  air  will 
circulate  freely  between  them.  Because  of  this  fact  it  is  possible  for 
air  to  reach  down  to  greater  depths  in  the  soil.  This  means  that  proper 
conditions  are  produced  at  greater  depths  in  the  soil  for  plant  root 
development  and  bacterial  growth.  The  deeper  penetration  of  roots 
and  soil  bacteria  again  means  that  organic  matter  and  nitrogen  will  be 
found  at  greater  depths  in  the  soil,  from  the  decay  of  dead  roots.  All 
these  agencies,  together  with  air,  will  work  toward  making  the  same 

(1) 


kind  of  soil  at  several  feet  below  the  surface  as  it  is  at  the  surface ;  the 
same  in  color,  fineness  or  coarseness,  amount  of  soluble  plant  food,  and 
bacterial  activity.  These  are  the  reasons  for  the  depth  of  California 
soils  in  their  important  bearings,  as  shown  by  scientific  investigations. 

What  has  been  said  above  has  an  important  bearing  on  our  methods 
of  tilling  soils  in  California.  For  if  the  soil  is  just  the  same  one  foot 
in  depth  as  it  is  at  the  surface,  why  not  make  use  of  the  fertility  of  it 
instead  of  confining  the  roots  of  our  plants  to  a  thin  layer  of  soil?  We 
have  nothing  to  fear  from  turning  the  deep  soil  up,  because  it  supports 
plant  growth  as  well  as  the  surface  soil,  as  is  evidenced  by  newly  graded 
soil  in  alfalfa  fields  and  orchards  in  California  where  at  times  two  and 
three  feet  of  the  surface  soil  are  removed  in  grading.  This  is  not  true 
in  soils  of  the  East,  because  clay  forms  rapidly  there  under  the  heavy 
leaching  action  of  continuous  rainfall  on  feldspar  minerals,  and  pro- 
duces a  cold,  light  colored  and  inactive  subsoil  on  which  plants  will 
not  grow  until  it  has  been  weathered  for  a  year  or  two. 

It  is,  therefore,  not  only  safe  to  plow  deeply  in  California,  but  it  is 
necessary  for  the  maintenance  of  fertility,  because  a  large  and  deep  root 
system,  the  formation  of  which  it  encourages,  will  cover  a  large  surface 
of  soil  particles  and  thus  have  a  larger  amount  of  food  and  water  at  its 
disposal.  Deep  plowing,  therefore,  is  the  first  requisite  to  successful 
agriculture  in  California  from  the  standpoint  of  soil  management. 

HOW  DEEP  TO  PLOW. 

By  deep  plowing  we  do  not  mean  the  same  plow  depth  for  every  crop 
and  soil.  The  following  are  general  rules,  which  will  serve  as  an 
efficient  guide.  For  orchard  crops  plow  eight  or  nine  inches ;  for  grain, 
8  to  10  inches ;  in  preparing  land  for  alfalfa  12  to  14,  or  even  16,  inches. 
For  vegetables  at  least  10  to  12  inches  are  necessary,  and  for  lima  beans 
and  sugar  beets  12  to  14  inches  will  be  better. 

In  general,  a  shallower  plowing  will  do  for  the  crops  above  mentioned 
on  sandy  soils,  but  it  is  important  to  plow  deeply  silty  clays,  clay  adobes, 
and,  in  general,  soils  which  "run  together"  easily  and  bake. 

WHEN  TO  PLOW. 

Plow  as  early  as  possible  in  the  fall  preceding  the  planting,  for 
potatoes,  lima  beans,  beets ;  also  in  preparing  land  for  alfalfa  and  for 
grain.  If  the  soil  is  not  too  heavy  and  will  allow  it,  prior  to  early 
rains,  plow  in  summer — especially  in  "summer-fallowing"  for  grain. 

In  orchard  practice  it  is  best  to  plow  in  spring,  and  the  plowing  here 
will  serve  also  for  turning  under  the  cover  crop  where  that  is  used. 
Plowing  may  also  be  done  in  summer  or  early  fall  in  the  orchard  if  the 
soil  is  very  heavy,  and  can  then  serve  as  a  preparation  for  seeding  the 
cover  crop.  On  most  soils,  however,  the  spring  plowing  is  sufficient  for 
all  purposes  of  aerating  the  soil  and  of  turning  under  various  fertilizing 
materials  and  weeds. 


CHANGE  THE  PLOW  DEPTH  EVERY  YEAR. 

One  of  the  causes  which  helps  to  form  plow  sole,  plowpan,  or  irriga- 
tion hardpan,  is  plowing  to  the  same  depth  every  year.  This  is  ex- 
plained in  another  circular,  but  it  suffices  to  say  here  that  we  must 
therefore  keep  the  subsoil  layers  from  becoming  hard  and  impervious. 
This  can  be  done  by  breaking  up  the  compacted  layer  formed  by  the 
sole  of  the  plow  in  plowing.  Such  breaking  up  can  be  accomplished 
by  changing  the  plow  depth  every  year.  Thus,  if  plowing  to  a  depth 
of  eight  inches  one  year,  plow  to  a  depth  of  nine  inches  the  next  year, 
and  go  back  again  to  eight  the  third  year.  In  most  cases  this  plan  will 
prevent  plow  sole  from  forming.  In  very  fine  grained  soils  a  subsoil 
plow  may  have  to  be  used  following  in  the  furrow  made  by  the  ordi- 
nary plow  to  accomplish  the  same  end. 

WHY  WE  CULTIVATE. 

Saving  moisture  in  California  soils  is  one  of  the  most  important 
factors  to  success  in  farming.  Keeping  good  air  conditions  in  the  soil 
is  another.  Both  of  these  things  can  be  accomplished  by  covering  the 
soil  with  a  blanket  or  mulch.  The  latter  cools  the  soil,  holds  the  water 
in,  and  prevents  the  soil  from  packing,  thus  keeping  up  good  air  con- 
ditions. Anything  that  covers  the  soil  will  serve  as  a  mulch.  But 
while  straw  and  manure,  which  are  the  most  effective  mulches,  can  be 
used  on  small  acreages,  they  are  too  expensive  and  involve  too  much 
labor  for  large  acreages.  For  the  latter,  therefore,  we  have  found  that 
we  can  make  a  very  efficient  blanket  or  mulch  out  of  the  upper  layer 
of  the  plowed  soil,  by  constantly  cultivating  it.  This,  frequently 
stirred,  soil  layer,  which  is  loose  and  dry,  acts  like  a  layer  of  straw  or 
manures.     It  is  known  as  a  dust  mulch. 

Constant  stirring  is  absolutely  necessary  to  maintain  a  useful  mulch. 
Wetting  of  the  upper  soil,  especially  by  rain,  compacts  it,  and  the  water 
evaporates  from  it  very  rapidly.  Therefore,  a  soil  surface  wetted  by 
rain  or  irrigation  water  must  be  cultivated  as  soon  as  it  will  not  be 
puddled  when  stirred  by  the  cultivator.  But  even  when  soil  is  not 
wetted  it  tends  to  become  more  and  more  compacted  from  the  weight 
of  its  own  particles.  Therefore,  whether  dry  or  wet  constant  stirring 
of  the  surface  soil  in  summer  is  imperative. 

WHEN  AND  HOW  TO  CULTIVATE. 

In  orchards  where  heavy  crops  of  fruit  are  borne,  the  writer  would 
recommend  cultivating  every  two  weeks  between  the  plowing  under  of 
the  cover  crop  and  the  next  sowing  of  it.  Where  no  cover  crop  is 
growing,  cultivating  should  be  done  with  the  same  frequency  through- 
out the  spring  and  summer.  This  practice  must  be  particularly  empha- 
sized in  orchards  grown  without  irrigation. 

In  cultivating  vegetables  more  frequent  cultivation  than  that 
employed  in  orchards  is  strongly  recommended.  It  is  always  to  be 
remembered  that  the  cost  of  cultivation  in  different  districts  may  vary 


—  4  — 

enough  to  make  it  advisable  in  some  eases  to  cultivate  more  frequently 
than  above  recommended,  and  in  other  cases  less  frequently.  The  more 
frequently  cultivation  is  carried  out  on  summer-fallowed  land  consistent 
with  the  income  from  the  same,  the  better.  The  writer  would  recom- 
mend at  least  one  cultivation  every  three  or  four  weeks. 

With  reference  to  the  depth  of  cultivating  California  soils  it  must  be 
said  that  careful  experiments  have  shown  a  5-inch  depth  to  be  about  the 
best.  Six  inches  may  be  used  where  more  than  a  10-inch  plow  depth  is 
employed. 

IMPLEMENTS  TO  USE  IN  PLOWING  AND   CULTIVATING. 

The  best  tool  for  plowing^  so  far  as  the  type  is  concerned,  is  the  mold- 
board  plow.  Any  form  of  it,  simple  or  compound,  as  suited  to  the  size 
of  the  ranches  or  the  flat,  rolling,  or  hilly"  nature  of  the  land  will  accom- 
plish the  work  better  than  other  forms  of  plows.  For  some  purposes 
the  disc  plow  will  serve  best,  however,  and  especially  for  very  deep 
plowing,  as  in  the  case  of  preparing  land  for  alfalfa,  a  disc  plow  of  the 
Spaulding  type  will  be  particularly  efficient. 

For  cultivators  use  the  lightest  implements  that  will  accomplish  the 
task  consistent  with  strength.  The  disc  cultivators  and  harrows  serve 
very  well,  but  in  fine  grained  soils  the  chisel  cultivators  will  be  found 
very  efficient  aids  to  the  discs  because  they  penetrate  very  deeply  and 
serve  to  aerate  the  soil  and  keep  it  from  packing.  Light  tools  are 
recommended  because  they  will  not  pack  the  soil  so  readily  and  help 
to  form  plow  sole.  For  the  same  reason  as  few  animals  as  possible 
should  be  used  in  plowing  and  cultivating  as  will  efficiently  and  easily 
do  the  work. 

SUMMARY. 

1.  Proper  plowing  and  cultivating  are  essential  factors  in  successful 
soil  management.     Reasons  are  given  above. 

2.  Deep  plowing  is  a  prime  necessity  in  California  soils.  Eight  to 
ten  inches  in  depth  is  right  for  ordinary  crops.  Fourteen  to  sixteen 
inches  in  preparing  land  for  alfalfa.     Details  given  above. 

3.  The  plow  depth  must  be  varied  slightly  every  year  to  help  prevent 
plow  sole  formation. 

4.  Constant  cultivation  is  important  on  California  soils.  Once  in 
two  weeks  in  orchards  or  oftener.  About  once  a  month  on  summer- 
fallowed  land. 

5.  The  ordinary  mold-board  plow  is  best  for  plowing.  Its  shape 
must  be  suited  to  the  topography  and  heaviness  or  lightness  of  soil. 

6.  Disc  harrows  and  cultivators  serve  best  for  our  silt  and  adobe  soil. 
The  ordinary  tooth  cultivator  will  do  for  very  light  soils.  See  details 
above. 


